Adrenal glands

General characteristics and embryogenesis

Externally, the adrenal gland is covered by a dense connective tissue capsule. Thin septa of loose connective tissue carrying blood vessels and nerves extend inward from the capsule, while the secretory cells themselves are surrounded by a network of reticular fibers. A cross-section clearly reveals two parts: a superficial yellowish cortex and a dark, soft medδulla.

This anatomical duality is the result of embryonic development. In lower vertebrates, these structures exist as separate glands, but in humans, they are united by a common blood supply.

• The cortex is of mesodermal origin. It forms from the coelomic epithelium, initially developing into the interrenal body and fetal cortex (which degenerates after birth), and subsequently forming the definitive cortex.
• The medulla is of neuroectodermal origin. Neural crest cells migrate together with sympathetic ganglion precursors, differentiate into chromaffinoblasts, and invade the interior of the future cortex.

Structure of the cortex

Cortical endocrinocytes specialize in the synthesis of corticosteroids, which are steroid hormones (cholesterol derivatives). These cells are characterized by a well-developed smooth endoplasmic reticulum (where hydroxylation occurs) and mitochondria with tubular cristae. The cortex is divided into three zones:

1. Zona glomerulosa (zona glomerulosa). Accounts for 15% of the volume, lying immediately beneath the capsule. Cells are arranged in rounded arches ("glomeruli"). It produces mineralocorticoids (e.g., aldosterone), which promote sodium and water retention in the kidneys. The activity of this zone is controlled by the renin-angiotensin system.
2. Zona fasciculata (zona fasciculata). The widest part (78%). Cells are arranged in long cords. They synthesize glucocorticoids (cortisol, corticosterone), which are essential for survival during chronic stress (they elevate blood glucose and mobilize resources). The cytoplasm of these cells is packed with lipids, making the zone bright orange when stained with Sudan III.
3. Zona reticularis (zona reticularis). The deepest and thinnest zone (7%). It produces androgens (androstenedione), which stimulate anabolic processes. The cells here are small and form a loose network.

Medulla and blood supply features

The medulla consists of cords of large basophilic cells. The bulk of it is composed of medullary endocrinocytes (chromaffin cells), which belong to the APUD system. Among them are:

• A-cells (the majority) — synthesize adrenaline (epinephrine).
• N-cells — synthesize noradrenaline (norepinephrine).

These catecholamines accumulate in secretory granules and are released into the blood during acute stress, triggering a systemic response similar to sympathetic nervous system activation (breakdown of fats and carbohydrates). Ganglion cells (about 1%) and supporting glial elements are also found in the medulla.

Angioarchitecture features: The blood supply to the gland is centripetal. Arterial blood from the capsule flows through the entire cortex via sinusoidal capillaries with fenestrated endothelium, collecting steroid hormones. This blood then enters the venous sinusoids of the medulla. Thus, chromaffin cells are constantly bathed in blood with a high concentration of glucocorticoids.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• The mechanism of activation of the renin-angiotensin-aldosterone system (RAAS) and its effect on the zona glomerulosa.
• Comparative ultrastructural characteristics of cells in the zona glomerulosa, zona fasciculata, and zona reticularis.
• The affiliation of medullary endocrinocytes with the APUD series: synthesis of regulatory peptides.
• Metabolic effects of glucocorticoids during chronic stress and their permissive action on the heart.

Other topics in the "Endocrine System" section

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